Hello,
I have a Kobo Aura One, and koreader [koreader-kobo-arm-kobo-linux-gnueabihf-v2015.11-1654-g666c9f1_2018-04-12]

My problem is that only some epub files I have put into place will show thumbnails for the covers in the KOReader File Browser.

epubs that I converted from amazon books are showing the covers, however comics / manga that I am converting myself from image files to epub are not showing up, even after I've used "polish" in calibre to embed covers.

The covers are embedding correctly evidenced by the fact that the covers I'm choosing work inside nickel (the kobo interface); However the same files do not show covers in the koreader file browser. I thought it was perhaps a file size issue, but I tried a single page of a comic as epub with an embedded cover that is a smaller file size than my books, however the cover still did not show.

Any ideas? I know it isn't a huge deal, but since I am anal about media management its really bothering me.

@cbutters: This won't be particularly helpful, but if you're using Calibre's Send To Device, the thumbnails are generated by Calibre, not Nickel, which may explain some inconsistencies.

I'm not familiar with this bit of KOReader's code, but since you mentioned mangas, I'm going to go out on a limb there and say that there may be a size/memory limitation at play somewhere. That wouldn't be the first time something like that has cropped up when talking about covers & mangas...

If I drop the cover into the source images before doing the conversion, and also use cbz instead of epub; I'm having good results. Had to do some legwork to convert and reimport several titles in my calibre library, but The added bonus is that I've found that switching pages with CBZ files in koreader is much faster than epub files for the same source material.

So no, I still don't know what is causing calibre embedded images on epub files to not work with koreader; but am using an effective workaround.

I think the calibre cover file you get when "update metadata" in calibre is only a local file based in /path_to_calibre libary/author/title/cover.jpg which is read by calibre using his metadata.opf which lives at the same place. When you convert, the book inner files are modified, so the cover is embedded inside.

Given the variety of archived image file/comic book, formats in use, as well as KOReader's current support for compressed material both archives and compressed image formats, adding CBT would seem reasonable. Displaying a sequence of image files from an archive that is not compressed should not be difficult. I don't see any reason it should replace any of the others, though. This isn't a situation where we need to pick between "Beta or VHS", we can have both, with little cost.